Protein Synthesis Reading

Name: ___________________________________________________________________ Period ______________ Date _________________

Protein Synthesis Reading

In the Genetics unit so far, we have discussed the following: How different people have different traits Our traits are determined by the kinds of proteins in our bodies DNA is a molecule inside each cell's nucleus, and is an extremely important molecule because it provides all the information that a cell or organism needs to live and thrive.

How do cells know which proteins to make, and when to make them? The answer lies within the DNA. The sequence of nitrogen base pairs (A, T, C, G) in a DNA molecule is the genetic code, which holds the instructions for how to make all the proteins found in our bodies.

DNA is safely protected within the nucleus of all cells. Different segments of DNA that code for different proteins are called genes. But cells have a problem: the raw materials they need to make proteins, amino acids, are found in the cytoplasm, not in the nucleus. Somehow, the instructions from the DNA need to be transferred into a form that can be carried to the cytoplasm of the cell.

Egyptian Hieroglyphs Analogy Think of DNA as a bunch of stone tablets that are permanently stowed inside an Egyptian pyramid. The tablets have hieroglyphs carved into them, which contain important information on how to run an ancient community. Because they are permanently inside the pyramids, most people cannot get to them and they can't read them either because they are written in a way that most people cannot understand. Someone who can read hieroglyphs, like a scribe, would copy the information on the tablet onto a piece of paper and then translate the information the tablet for everyone else to understand.

The "Central Dogma" Cell needs a way to transfer information from DNA into a readable form. It does so by using a molecule called RNA. Only through translation of the DNA message into RNA and protein does the information in DNA become actually useful. RNA and proteins take the script (DNA) and bring it to life.

The cell follows two main steps to make a protein encoded by DNA. These two steps are so important that they are often called the "central dogma" of molecular biology. The central dogma is DNA RNA Protein and the steps are:

1. Transcription is when the cell makes copy of the information onto an messenger RNA molecule (called mRNA). 2. Translation is when the message present in the mRNA is translated into an entirely new language using amino

acids, a protein.

Two Main Types of RNA DNA is located inside the nucleus of the cell and contains the instructions to create proteins but does not make the protein itself, kind of like how an architect will design a building, but he or she will not build that building by their self. Proteins are actually made by ribosomes located in the cytoplasm. So how do the instructions on the DNA get to the ribosome? DNA sends out a message in the form of RNA (ribonucleic acid), describing how to make the protein.

There are two main types of RNA directly involved in protein synthesis: 1. Messenger RNA (mRNA) carries the instructions from the nucleus to the cytoplasm. 2. Transfer RNA (tRNA) is involved in the process of ordering the amino acids to make the protein.

Both RNAs are nucleic acids, made of nucleotides, similar to DNA. The RNA nucleotide is different from the DNA nucleotide in the following three ways:

1. RNA contains a different kind of sugar, called ribose. 2. In RNA, the base uracil (U) replaces the thymine (T) found in DNA. 3. RNA is a single strand.

TRANSCRIPTION The process of constructing an mRNA molecule by using DNA as a template is known as transcription (See Figures A and B). The double helix of DNA unwinds and the nucleotides follow almost the same base pairing rules to form the correct sequence in the mRNA. The exception is that uracil (U) pairs with each adenine (A) in the DNA instead of thymine pairing with adenine. In this manner, the genetic code is passed on to the mRNA.

Figure A: Each gene (a) contains triplets of bases (b) that are transcribed into RNA (c). Every triplet, or codon, encodes for a

unique amino acid.

Figure B: Base-pairing ensures the accuracy of transcription. Notice how the helix must unwind for transcription to take place.

TRANSLATION The mRNA made during transcription is directly involved in the protein-making process. mRNA tells the ribosome (Figure C) how to create a protein. The process of reading the mRNA code in the ribosome to make a protein is called translation (Figure D). Sets of three bases, called codons, are read in the ribosome, the organelle responsible for making proteins.

Figure C: Ribosomes translate RNA into a protein with a specific amino acid sequence. The tRNA binds and brings to the ribosome the amino acid encoded by the mRNA. Ribosomes are made of rRNA and proteins.

The following are the steps involved in translation:

1. mRNA travels to the ribosome from the nucleus.

2. The code in the mRNA determines the order of the amino acids in the protein. The genetic code in mRNA is read in "words" of three letters (triplets), called codons. There are 20 amino acids and different codons code for different amino acids. For example, GGU codes for the amino acid glycine, while GUC codes for valine.

3. tRNA reads the mRNA code and brings a specific amino acid to attach to the growing chain of amino acids. Each tRNA carries only one type of amino acid and only recognizes one specific codon.

4. tRNA is released from the amino acid after the amino acid bonds to another amino acid with a peptide bond.

5. Three codons, UGA, UAA, and UAG, indicate that the protein should stop adding amino acids. They are called "stop codons" and do not code for an amino acid. Once tRNA comes to a stop codon, the protein is set free from the ribosome.

Figure D: This summary of how genes are expressed shows that DNA is transcribed into RNA, which is translated in turn to protein.

Analysis Questions 1. Where does transcription take place? _____________________________________________ 2. Where does translation take place? _____________________________________________ 3. What is made in the process of transcription? ___________________________________________ 4. What is made in the process of translation? _____________________________________________ 5. The "m" in mRNA stands for "messenger". Why? What is its role?

6. Transcribe the following DNA segment into mRNA: A A T A C A T G T T A G C C T T A A T C A A 7. What are codons?

8. If AUG is the starting codon in the mRNA and UAG is the stop codon, list the other codons in the sequence in #6.

9. What role does tRNA play in protein synthesis?

10. What would happen to the protein if one of the nucleotides in the sequence were accidentally changed to a different one during DNA replication? How do you know?

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